Method and device for tank leakage diagnosis at elevated fuel degassing

ABSTRACT

A method for operating a tank leakage diagnosis device, especially of a motor vehicle, volatile fuel (to be degassed) being temporarily stored using an adsorption filter of known absorption capacity or absorption characteristics, and the adsorption filter being regenerated from time to time by purging, using fresh air, to avoid faulty measurements in the tank leakage diagnosis, in particular at elevated fuel degassing. It is provided that the adsorption filter be purged, and, in this context, the volatile fuel removed from the adsorption filter over a predefined time span be integrated, and from that, the loading of the adsorption filter with the volatile fuel, changing during the time span, is ascertained, from the absorption capacity and the absorption characteristics of the adsorption filter, as well as the integrated fuel quantity or the changing loading. The quantity of fuel to be degassed supplied to the adsorption filter from the fuel container in the time span is calculated, and, as a function of the calculated quantity of fuel supplied to the adsorption filter, an intervention is undertaken at the tank leakage diagnosis device.

FIELD OF THE INVENTION

The present invention relates to a method for operating a tank leakagediagnosis device, particularly of a motor vehicle. In addition, thepresent invention relates to a control unit and a tank leakage diagnosisunit for carrying out the method.

BACKGROUND INFORMATION

In a fuel storage tank of a motor vehicle that contains fuel, volatilehydrocarbons are continuously escaping. This effect increases withtemperature and the agitation or sloshing of the fuel. In motor vehiclesdriven by internal combustion engines, for a flawless fuel supply,venting of the fuel storage tank is absolutely essential. For, as fuelis used up, air has to be able to flow in behind it, since otherwise avacuum would form in the tank, and the fuel flow would come to a stop.However, the tank also has to be vented so as to give the tank'scontents sufficient opportunity to expand as it warms up. Also, when thetank is filled up, sufficient air has to be able to exit the tank sothat the fuel being filled up does not bubble out of the filler pipeagain.

Therefore, in such vehicles, increasingly tank venting systems are usedin which the evaporating and excess fuel vapor is guided not into theopen air but, via a venting line, into an active charcoal filter (AKF).This fuel vapor is stored temporarily in the AKF, and, during theoperation of the motor vehicle, is guided via a clocked activatableelectromagnetic tank venting valve (TEV) to the intake manifold of theinternal combustion engine, and thus to combustion. This preventsemission of the environmentally harmful fuel vapors from the tank intothe environment to the greatest extent, and at the same time the vaporssupplied to the internal combustion engine are themselves used as fuel,whereby fuel usage is considerably reduced.

Based on the limited absorption volume of the active charcoal used inthe AKF, one should intermittently regenerate the AKF. In order to dothis, while the internal combustion engine is running, fresh air isdrawn in via the AKF, and the fuel vapor removed in the process issupplied to the internal combustion engine as a mixture for combustion.The respective flushing quantity is controlled by the TEV via aperformance characteristics adaptation using the parameters load androtary speed, so that the running properties of the internal combustionengine are not impaired. A lambda control additionally monitors andregulates the regeneration. The lambda deviation resulting from this canthen be drawn upon as a measurement of the loading state of the AKF.

In this connection, intensified legal regulations on the operation ofinternal combustion engines will apply in the future in some countries,such as the USA. Thereafter, it will be required for motor vehicles, inwhich volatile fuels like gasoline are used, that a possibly existingleakage in the entire fuel tank system be tracked down using an on-boardarrangement.

Corresponding methods and devices for tank leakage diagnosis in a tankventing system of a motor vehicle are referred to, for example, in theU.S. Pat. No. 5,349,935, DE 196 36 431.0 A1, DE 198 09 384.5 A1 and DE196 25 702 A1. In these, an overpressure is applied to the tank ventingsystem, and a conclusion as to the presence of a leak is drawn from thesubsequent course of the pressure. In the system of DE 196 36 431.0 A1,one may form a ram pressure between a pump and a reference leak, wherebythe pump's rotary speed is lowered and the pump's current consumptionincreases. If the tank is leakproof, a higher pressure develops thanwhen against the reference leak. The current consumption is consequentlyhigher.

It may be observed that the tank leak diagnosis, instead of by the useof overpressure, may also be performed with the aid of underpressure.

A relatively high fuel degassing leads to erroneous measurements in tankleakage diagnosis. Therefore, as a measure of increased fuel degassing,a filtered loading factor of the AKF is used as a basis. The loadingfactor is calculated during travel, and filtered via a time constant. Todo this, with the engine running, the TEV is controlled to open, and thedeviation coming about in the lambda regulator, in this context, isrecorded. Using the recorded deviation, together with the volume streamthrough the TEV that is also present in the engine control, thehydrocarbon (HC) concentration of the drawn in flushing volume stream iscalculated. The HC concentration of the air drawn in through the AKFthus ascertained is valid as the measure of the magnitude of the AKF'sloading. If the loading value exceeds a predefined threshold, theleakage diagnosis is interrupted or temporarily blocked.

Since the loading is a function not only of the magnitude of the fueldegassing, using the loading value alone no accurate statement can bemade concerning the actual magnitude of the instantaneous degassing.Thus, even at a very great fuel degassing under certain travelconditions, the loading factor may be artificially kept low using a highpurging rate. In such a case, the leakage diagnosis would be enabledbecause of the low loading factor and the low degassing supposed fromthis. In actual fact, however, because of the actually present highdegassing, this would lead to erroneous results in the leakagediagnosis. In the case of the overpressure diagnosis method discussedabove, the leakage quantities specified by law in the USA would not bemet. In the underpressure methods also named, such an erroneousdetection may lead to the mistaken diagnosis of a non-leakproof tanksystem.

SUMMARY OF THE INVENTION

Therefore, the exemplary method of the present invention is intended toavoid erroneous measurements in tank leakage diagnosis, particularly atelevated fuel degassing.

The exemplary embodiment and/or exemplary method of the presentinvention is based on ascertaining the actual instantaneously presentfuel degassing, and, as a function of the ascertained degassing value,of suppressing affected diagnosis functions in order thereby to avoidfalse diagnoses. According to one variant, a substantial improvement inthe quality of the diagnosis may be brought about, depending on thediagnosis function affected, by compensation of the disturbance measuredby the degassing present during the tank leakage diagnosis.

For this, the exemplary method according to the present inventionprovides that the adsorption filter be flushed, and in this context, thevolatile fuel removed over a predefined time span from the adsorptionfilter be integrated, and from that there be ascertained loading of theadsorption filter with the volatile fuel that changes within the timespan, that from the adsorption capacity or adsorption characteristics ofthe adsorption filter, the loading factor made available as well as theintegrated fuel quantity or the changing loading, the quantity ofdegassing fuel supplied to the adsorption filter from the fuel containerin the time span be calculated, and, as a function of the calculatedquantity of the fuel supplied to the adsorption filter, an interventionis undertaken at the tank leakage diagnosis unit. According to this, abalance calculation is carried out or performed from which a conclusionis drawn on the fuel mass supplied to the adsorption filter from thefuel mass removed during the purging of the adsorption filter. In thiscontext, the fuel mass supplied to the adsorption filter is assumed tobe the actual degassing mass.

In a first variant it is provided that at least one leakage diagnosisfunction is interrupted or blocked as a function of the calculatedquantity of degassing fuel. According to a second variant, also as afunction of the calculated quantity of degassing fuel, there takes placean immediate, or possibly time-delayed, compensation of the disturbancein the tank leakage diagnosis conditioned upon the calculated quantityof degassing fuel.

The interventions, at the tank leakage diagnosis device mentioned, takeplace either in response to each present calculated value, in the way ofa compensation, or in each case only when the calculated quantity ofdegassing fuel exceeds a predefinable threshold value.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a fuel tank system of a motor vehicle, in which theexemplary method according to the present invention is used.

FIG. 2 shows a diagrammatic representation of the time characteristic ofthe AKF loading at different AKF purging streams.

FIG. 3 shows a functional sequence of a control unit according to anexemplary embodiment and/or method of the present invention, as a flowdiagram.

DETAILED DESCRIPTION

FIG. 1 shows an intake manifold 10 that may be provided in a (not shown)internal combustion engine (BKM) especially of a motor vehicle, as wellas an exhaust gas tract 12. A fuel storage tank 14 is provided for fuelstorage.

For the low-emission operation of the BKM, there are provided a tankventing device 16, a control unit 18, an exhaust gas sensor system 20,as well as a sensor system 22, which takes the place of a plurality ofsensors ascertaining operating parameters of the BKM, such as a rotaryspeed sensors, flow meters for sensing the intake air quantity,temperature sensors, etc. The device shown also provides a fuel meteringdevice 24, which, for instance, may be implemented as equipment for oneor more injection valves.

Tank venting (ventilation) device 16 includes an active charcoal filter(AKF) 26, which communicates via corresponding lines 28-32 with tank 14,environmental air 34 and intake manifold 10 of BKM. The correspondinggas flow directions are indicated by arrows. In line 32 going to intakemanifold 10, there is a tank venting valve (TEV) 36.

AKF 26 stores just evaporating fuel in tank 14. As TEV 36 is opened bycontrol device 18, air 34 is drawn in from the environment through AKF26, which at the same time releases the stored fuel into drawn-in air34. This fuel/air mixture denoted as “tank venting mixture” or also as“regenerating gas” (called “HC mass” below) now influences thecomposition of the gas mixture supplied altogether to the BKM, which isalso determined by a metering of fuel adjusted to drawn-in air quantity34 via fuel metering device 24.

In this context, in extreme cases, the fuel supplied to intake manifold10 via tank ventilating system 16 may constitute a proportion ofapproximately one-third to one-half of the entire fuel quantity.

During the operation of the motor vehicle or the BKM, or during fillingup tank 14, volatile hydrocarbon vapors (HC vapors) form in tank 14,which get into AKF via line 28 and are reversibly bound in it in a knownmanner. TEV 36 is normally closed. At regular time periods, TEV 36 iscontrolled via control unit 18 in such a way that a certain partialpressure of the underpressure existing in intake manifold 10 is suppliedto AKF 26 via line 32, which leads to the stored HC vapors being drawnfrom AKF 26 via line 32 and via TEV 36 into intake manifold 10, so asfinally to be supplied to BKM for combustion and thus to final disposal.During this process of regeneration of AKF 26, purge air 34 is drawninto AKF26 via line 30, and possibly via a passive filter, whereby theactual purging effect is effected.

For an onboard diagnosis of the operability or tightness of tank 14 orthe entire tank system, a leakage diagnosis unit 40 is provided that isconnected to tank 14 via a line 38. Leakage diagnosis unit 40 andcontrol unit 18 may be integrated into a single control unit (notshown). Leakage diagnosis unit 40 has a pump 42, which has a switchingvalve 44 connected upstream of it. A reference leak 46 is situatedparallel to line 38. The magnitude of reference leak 46 is selected sothat it corresponds to the magnitude of the tank leak that is to berecorded. Switch-over valve 44 has two switch settings. In the firstsetting, pump 42 is connected to line 48 in a pressure-conductingmanner, and then pumps external air 50 all the way through referenceleak 46 into line 48. A micro-filter 52 is connected upstream ofreference leak 46 towards the outside so as to prevent reference leak 46becoming blocked by particles possibly drawn in.

The onboard diagnosis named is discussed in detail, for example, in DE196 36 431.0, to which reference is made and which is incorporated byreference as necessary.

The premise on which the exemplary method is based will now be shown inthe light of FIG. 2. This is made up of a balance calculation which isbased on the premise that a fluid level that sets in in the AKF withrespect to loading depends, on the HC quantity which is removed from theAKF via the purge stream of the tank ventilation, and on the HC quantitywhich is supplied to the AKF by the degassing of the fuel. The timecharacteristic during the transition from the one to the other fluidlevel on account of a changed degassing or purging quantity depends, inturn, on the adsorption capacity of the AKF.

If, over a certain time span, more HC is removed from the AKF than flowsin from the tank, then its fluid level decreases. If, over a certaintime span, less HC mass is removed than flows in from the tank, then itsfluid level increases. If removal via the tank ventilation and supplyfrom the tank are approximately in balance, then the fluid level alsoremains nearly constant.

Consequently, as the balance equation one may write:AKF-HC-loading=the integral(tank_HC-degassing)−the integral(AKF_HC-venting)

The loading factor ‘ftead’ required for the balance calculationmentioned is already available as the calculated magnitude in the tankventilation function.

FIG. 3 illustrates exemplary functional sequences of a control unitaccording to the exemplary embodiment and/or exemplary method of thepresent invention. After start 100 of the routine, a purging of the AKFis brought about 102 via the tank ventilation, and at the same time atimer is started 104 having a zero reading. After that, theinstantaneous engine load of BKM is recorded 106. Based on the recordedload, a calculation 108 is carried out of the instantaneous HC massburned in the BKM. At the same time as the steps mentioned, in the lightof a lambda regulation 110 it is determined 112 whether a mixtureadaptation carried out in the light of the lambda regulation has builtup. If not, an increased or high HC degassing in the tank is assumed114, and as a result, at least one diagnosis function is blocked 116, ora perhaps already present blocking is confirmed. Alternatively oradditionally, by corresponding overcompensation of the actually lowerdegassing a false diagnosis may be avoided at the leakage diagnosis unitin the way of a correction. If the mixture adaptation has already builtup, it is further checked 118 whether a mixture deviation is present. Ifnot, the system goes back to start 100. Otherwise, mixture correctingfactors are recorded 120 that were supplied by the lambda regulation orthe engine control

The HC quantity removed from the AKF via the purge stream is calculatedas exactly as possible. For that purpose, the mixture deviationsmentioned, which do not originate from the purge stream of the tankventilation, are avoided. As in the example, this may occur by waitinguntil the mixture adaptation has built up. From the fuel calculated inthe load recording and required for combustion, and the mixturecorrection factors from the lambda regulation and the tank ventilatingfunction, the HC mass removed from the AKF via the tank ventilation iscalculated in this context and integrated over time.

If a calculation is not possible, the exemplary method is continued withusing above-named step 114. Optionally, the further procedure can bemade dependent on the mixture adaptation having already built up andthere is a mixture deviation. This may be implemented by correspondingflags at steps 112 and 118. If this is the case, the HC mass removedfrom the AKF is calculated 124, taking under consideration the recordedmixture correction factors 120, as well as the AKF loading factor‘ftead’ 126, that has changed in the meantime, and the predefined AKFstorage capacity or storage characteristics 128.

The calculated values of the removed HC mass are subsequently integratedover time 130, the instantaneous time calculated by timer 104 in eachcase being taken as the total time.

In the exemplary embodiment, loading factor 126 is additionally stronglylowpass-filtered to record the instantaneous “AKF liquid level”, underthe assumption that the actual AKF loading changes only slowly overtime. Alternatively, the liquid level may be recorded only atsufficiently constant operating conditions. This should at least betterensure independence from travel-dynamic influences.

The storage capacity of the AKF as well as its “characteristics”, i.e.the HC release versus fluid level, and possibly also the dependence offurther parameters, such as temperature, or the like, are available, sothat using loading factor 126 one may conclude what the AKF fluid levelis.

Thereafter, the HC mass supplied to the AKF is calculated from thebalance equation (FIG. 2) mentioned 132. This HC volume stream from thetank corresponds to the degassing in the tank. If the calculated HC masssupplied to the AKF exceeds a predefined threshold 134 that is to beempirically determined, then the at least one diagnosis function isblocked 116 or a previously described compensation is carried out orperformed by the diagnosis itself.

It should be emphasized that the function cycle shown may, as a rule,run through several times, as indicated by broken line 136, and thevalues ascertained in each case of the HC mass removed from the AKF areintegrated in each case, in this context. In this context, temporalinterruptions do not make any difference.

1-16. (Canceled).
 17. A method for operating a tank leakage diagnosisdevice in a motor vehicle, for testing a fuel container, which isconnected to an internal combustion engine, for tightness, the methodcomprising: regenerating an adsorption filter from time to time bypurging by fresh air drawn in by the internal combustion engine, whereindegassing fuel is temporarily stored with an adsorption filter having atleast one of an adsorption capacity and adsorption characteristics, theadsorption filter being connected to the fuel container; providing aloading factor specifying the loading of the adsorption filter with thedegassing fuel, wherein the adsorption filter is purged and, in thiscontext, the degassing fuel removed from the adsorption filter over apredefined time span is recorded; determining, from at least one of theabsorption capacity, the absorption characteristics of the adsorptionfilter, the loading factor, a recorded, removed fuel quantity and achanging at the loading, a quantity of the degassing fuel supplied tothe adsorption filter from the fuel container over the predefined timespan; and performing, as a function of a determined quantity of thedegassing fuel supplied to the adsorption filter, at least one of: (i)one of an interrupting and a blocking of at least one tank leakagediagnosis function, and (ii)a correcting of the data as to an actualtank degassing ascertained with the at least one tank leakage diagnosisfunction.
 18. The method of claim 17, wherein one of the following issatisfied:(i) the at least one tank leakage diagnosis function is one ofinterrupted and blocked only when a determined quantity of the degassingfuel supplied to the adsorption filter exceeds a predefinable thresholdvalue; and (ii) the data with respect to the actual tank degassing,ascertained with the at least one tank leakage diagnosis function, iscorrected only when the determined quantity of the degassing fuelsupplied to the adsorption filter exceeds a predefinable thresholdvalue.
 19. The method of claim 17, wherein the degassing fuel removedfrom the adsorption filter over the predefined time span is recorded inan integrating manner.
 20. The method of claim 17, wherein a balanceequation of the form AKF_HC-loading=the integral(tank_HC-loading)−theintegral(AKF_HC-venting) is used to determine the loading factor. 21.The method of claim 17, wherein an exceeding of a defined threshold isassumed to be satisfied until it is possible to determine the quantityof the degassing fuel within a predefined tolerance during operation ofthe internal combustion engine.
 22. The method of claim 17, wherein aquantity of temporarily stored degassing fuel removed from theadsorption filter by purging is ascertained from at least one of: (i) afuel quantity required for combustion is determined in a load sensing ofthe internal combustion engine, and (ii) by taking, as a basis, amixture correction factor resulting from a lambda regulation.
 23. Themethod of claim 17, wherein the quantity of the degassing fuel isrecorded only at sufficiently constant operating conditions of theinternal combustion engine.
 24. The method of claim 17, wherein a valueof the loading factor of the adsorption filter is low-pass filtered. 25.A control apparatus for operating a tank leakage diagnosis device in amotor vehicle, for testing a fuel container, which is connected to aninternal combustion engine, for tightness, degassing fuel beingtemporarily stored with an adsorption filter that has at least one of anadsorption capacity and adsorption characteristics, connected to thefuel container, comprising: a regenerating arrangement to regenerate theadsorption filter from time to time by purging fresh air drawn in by theinternal combustion engine; a loading factor arrangement to provide aloading factor specifying a loading of the adsorption filter with thedegassing fuel; a recording arrangement to record engine characteristicsdata and the loading of the adsorption filter with the degassing fuel,and for determining the degassing fuel supplied to the adsorption filterfrom the degassing fuel removed from the adsorption filter and theloading; and an arrangement for one of: (i) one of blocking andinterrupting at least one tank leakage diagnosis function; and (ii) forcorrecting data as to an actual tank degassing, ascertained with atleast one tank leakage diagnosis function, as a function of a determinedquantity of degassing fuel supplied to the adsorption filter.
 26. Thecontrol apparatus of claim 25, wherein the blocking arrangement includesa comparing arrangement to compare a determined amount of emitted gas,supplied to the adsorption filter, to a predefined threshold value, andwhen the predefined threshold value is exceeded, performing the one of(i) the blocking and interrupting and (ii) the correcting.
 27. Thecontrol apparatus of claim 25, further comprising: a timer; anintegrator for integrating calculated values of the degassing fuelsupplied to the adsorption filter; and a starting arrangement toactively start a purging of the adsorption filter.
 28. The controlapparatus of claim 25, further comprising: a testing arrangement to testwhether a calculation of the emitted gas removed from the adsorptionfilter is possible, based on recorded operating variables of theinternal combustion engine.
 29. The control apparatus of claim 25,further comprising: a filtering arrangement to low-pass filter therecorded loading of the adsorption filter.
 30. A tank leakage diagnosisapparatus comprising: a tank leakage diagnosis device for a motorvehicle, for testing a fuel container, which is connected to an internalcombustion engine, for tightness, the device performing the following:regenerating an adsorption filter from time to time by purging by freshair drawn in by the internal combustion engine, wherein degassing fuelis temporarily stored with an adsorption filter having at least one ofan adsorption capacity and adsorption characteristics, the adsorptionfilter being connected to the fuel container; providing a loading factorspecifying the loading of the adsorption filter with the degassing fuel,wherein the adsorption filter is purged and, in this context, thedegassing fuel removed from the adsorption filter over a predefined timespan is recorded; determining, from at least one of the absorptioncapacity, the absorption characteristics of the adsorption filter, theloading factor, a recorded, removed fuel quantity and a changing at theloading, a quantity of the degassing fuel supplied to the adsorptionfilter from the fuel container over the predefined time span; andperforming, as a function of a determined quantity of the degassing fuelsupplied to the adsorption filter, at least one of: (i) one of aninterrupting and a blocking of at least one tank leakage diagnosisfunction, and (ii)a correcting of the data as to an actual tankdegassing ascertained with the at least one tank leakage diagnosisfunction.
 31. A tank leakage diagnosis apparatus comprising: a controlapparatus to control a diagnosing of a tank leakage in a motor vehicle,for testing a fuel container, which is connected to an internalcombustion engine, for tightness, degassing fuel being temporarilystored with an adsorption filter that has at least one of an adsorptioncapacity and adsorption characteristics, the adsorption filter beingconnected to the fuel container, the control apparatus including: aregenerating arrangement to regenerate the adsorption filter from timeto time by purging fresh air drawn in by the internal combustion engine;a loading factor arrangement to provide a loading factor specifying aloading of the adsorption filter with the degassing fuel; a recordingarrangement to record engine characteristics data and the loading of theadsorption filter with the degassing fuel, and for determining thedegassing fuel supplied to the adsorption filter from the degassing fuelremoved from the adsorption filter and the loading; and an arrangementfor one of: (i) one of blocking and interrupting at least one tankleakage diagnosis function; and (ii) for correcting data as to an actualtank degassing, ascertained with at least one tank leakage diagnosisfunction, as a function of a determined quantity of degassing fuelsupplied to the adsorption filter.